The present invention relates to a control system for a four-wheel drive vehicle for automatically changing the power transmission of the vehicle from two-wheel drive to four-wheel drive in accordance with road conditions.
In a conventional four-wheel drive vehicle, a power transmission system for a two-wheel driving is selectively changed to a four-wheel driving system by engaging a transfer clutch which is manually operated by a select lever.
For example, when the vehicle travels on slippery roads, such as snowy, sloppy or gravel road, by two-wheel driving, the transmission system should be changed to four-wheel drive in order to prevent the driving wheels from slipping. If this changing operation is done after the slipping occurs because of the driver's misjudgment of the slipping, the slipping cannot be stopped or reduced.
In order to resolve such a problem, an automatic control system for the four-wheel drive system has been proposed. The conventional system is provided with a slip detecting circuit which produces a clutch signal when the difference between the speed of the front and rear wheels exceeds a predetermined reference value. The clutch signal causes a clutch to engage, so that the transmission system is automatically changed to four-wheel drive.
However, since the automatic control system detects the slipping which already has occurred, the occurrence of slipping can not be prevented. Accordingly, such an automatic control system is not effective to prevent the slipping of wheels.